SMTP options for sending low volume emails reliably

I host a number of websites on my droplet that need to occasionally send emails for things such as password resets or sale acknowledgements. The volumes are very low - maximum of 100 per week but often none. I have previously used a free Gmail SMTP but it now seems that without SPF and DMARC that any emails sent to the likes of gmail or outlook addresses get rejected.

I tried setting up SendGrid but the second email I tried to send failed to be delivered because the IP address used by SendGrid was blacklisted at the time. The only suggestion from support to avoid this would be to purchase a PRO service at $90 per month with a dedicated IP address rather than use the IP pool. This suggested to me that all the major volume email providers are likely to suffer the same issue which might be OK if sending lots of marketing emails where it doesn’t matter if some fail to be delivered. Whereas my requirement is very low volume but reliable delivery.

I am sure that many DigitalOcean users must have a similar requirement so has anyone found a free or low cost reliable SMTP option?